Unless a man be born a magician, and God have destined him even from his birth to the work, so that the spirits do willingly come of their own accord — which doth happen to few — a man must use only of those things herein set down, or written in our other books of occult philosophy, as means to fix the mind upon the work to be done; for it is in the power of the mind itself that spirits do come and go, and magical works are done, and all things in nature are but as uses to induce the will to rest upon the point desired.
—Henry Cornelius Agrippa Von Nettesheim, Three Books of Occult Philosophy